Then, would consequently be immoral to enter into a marriage knowing that you are sterile?
Sterile people may marry. If the sterility is known it should be **known by both parties **not one.
In essence is marriage in the case of sterility only permitted as an accident?
**If both parties know **one party is sterile they can marry.
Does the same argument apply to post-menopausal women?
The argument concerns procreation and is not in play here. The confusion lies in understanding the normal (and designed) actions verses the abnormal (or subverted design) actions. If a man and woman enter a marriage covenant, and perform normally then results are to accepted. In some cases their union will be barren. In the cases mentioned here the union will be barren. It is still a union.
It is important to know should sterility be hidden from one partner that partner could petition for anullity based on the hidden knowledge of the sterility.